Natural gas hydrate stability conditions and water activity in aqueous solutions containing mono ethylene glycol (MEG) and salt: Experimental measurements and thermodynamic modeling

Abstract

Gas hydrates can form in the presence of saline water produced from the gas and oil reservoirs under specific thermodynamic conditions. One of the best ways to prevent gas hydrate formation is to inject hydrate inhibitors like mono ethylene glycol (MEG) or methanol aqueous solutions in the pipelines. To design productive facilities and overcome operation faults, studying the stability conditions of natural gas hydrates in the presence of aqueous solutions of salt(s) and/or inhibitor(s) is necessary. However, a few studies have been conducted on stability conditions of natural gas hydrates while salt mixtures and MEG are present in aqueous solution. Here, the hydrate stability conditions of a synthetic natural gas in aqueous mixtures of 20 and 30 wt% of MEG, and 2.5 wt% NaCl + 2.5 wt% KCl were experimentally measured under 272–293 K and 3.30–12.70 MPa. An isochoric pressure-search method was employed to conduct the measurements. Then, a thermodynamic model was developed to estimate hydrate stability conditions in the presence of the aforementioned inhibitors. In this developed model, the aqueous phase fugacity is estimated using a combined form of the Cubic-Plus-Association Equation of State with the modified Debye-Hückel electrostatic term and the hydrate phase is modeled using the van der Waals and Platteeuw solid solution theory. To improve model accuracy, the aqueous phase activity was measured in various concentrations at atmospheric pressure and 298.15 K using a standard activity meter called Novasina LabMaster-aw neoAS, Switzerland. These measured data were used to correlate the activity coefficient parameters in the thermodynamic model. Finally, it is noted that the modeling results and the experimental data are in satisfactory agreement.